This invention relates in general to antennas and, more specifically, to a monocoque constructed fully enclosed terrestrial antenna for receiving signals from satellites in geostationary orbit.
The first communications satellite, Telstar I, placed in geostationary orbit, 22,300 miles over the equator in 1962 could carry only one television signal or 12 telephone calls at one time. Today, many communications satellites are in geostationary orbit, each capable of handling 24 television signals. Originally, signals were fed to the satellite by an uplink dish at a television studio and beamed back to earth to receiving antennas at television broadcast stations. In 1976 the first home-built private receiving antenna was put into use. Since then, the home satellite market has grown rapidly, with hundreds of thousands of home antenna systems now in use worldwide.
A wide variety of antennas are in use, generally having diameters of from about 6 to 20 feet and made from many materials. Each system basically consists of a concave dish having the ability to reflect incoming signals to a feedhorn assembly positioned above the center of the dish, which collects the incoming signal, passes it through a low-noise amplifier and downconverter, then to a satellite receiver and finally to a conventional television set. The axis of rotation of the antenna must be pointed at the desired satellite in order to receive a signal therefrom. Both manually adjusted ground supports and polar mounts which can be manually cranked or motor driven are available.
The antenna dish may be made from metal, which may be in the form of a mesh, or an insulating material such as glass fiber reinforced plastic, thermoformable or moldable plastic, or other materials having a conductive layer or coating that may be sandwiched in or on the concave face. The feedhorn assembly is generally supported on a rod attached near the center of the antenna and extending out to a point above the antenna surface. The concave face is ordinarily exposed to the elements.
While these antennas generally produce acceptable results, a number of problems remain. Snow, rain, leaves and other foreign matter often collects on the upwardly-pointed concave antenna surface, degrading picture quality. Rain on the surface deflects the signal, since the signal must pass through the water twice. This is a particular problem with the "KU" (12 ghz) band.
When a dish is exposed to sunlight, with part of the concave surface exposed to sunlight and part in the shade, differential thermal expansion cause distortions in the surface, degrading picture quality.
Such antennas, being flat across the opening and having a convex underside, may produce an airfoil effect in some wind conditions, causing dish movement, vibration and possible destruction. Also, winds impinging directly on the feedhorn assembly at the end of a thin mounting rod may cause vibration adversely affecting picture quality. The thin unsupported dish surface may need to be heavy or have a number of supporting ribs to provide sufficient strength. Such antennas appear "busy" and unattractive.
Thus, there is a continuing need for satellite signal receiving antennas which overcome the above-noted problems and provide other advantages and features.